An image sensor converts an optical image into an electrical signal. Types of image sensors include charge-coupled devices (CCDs) and complementary-metal-oxide-semiconductor (CMOS) image sensors. Image sensors are commonly used in digital cameras as well as other imaging devices.
Conventional image sensors include pixels arranged in a plurality of rows and columns. Each pixel includes a photo sensor that accumulates charge when illuminated by light. Conventionally, pixels accumulate a charge in an analog circuit for a continuous period of time referred to as an integration time (also referred to as an exposure time). The accumulated charge is transferred to an analog-to-digital (A/D) converter, which converts the accumulated charge into a digital value for that pixel.
When a photo sensitive device is over exposed (i.e., exposed to more photons than the device is able to absorb and convert into electrons), the excess electrons may flow into adjacent pixels, which results in a halo effect around bright pixels. This is commonly referred to as blooming. Blooming can be a problem in conventional CMOS image sensors under conditions of relatively high illumination, in shared pixel structures, and in image sensors utilizing an electronic rolling shutter.
Blooming can also be an issue in conventional boundary or skipping cases where pixels that are not read out continue to accumulate charge continuously even under low illumination conditions. In this case, these pixels have a relatively high blooming risk.
In a shared pixel, for example, two or mode photodiodes and transfer transistors share the remaining portion of the pixel circuitry. In this case, there is a (relatively) low impedance flow path for electrons between the photodiodes. Consequently, blooming may occur more easily.
In an image sensor utilizing an electronic rolling shutter, when the image sensor is exposed to relatively high illumination, the exposure time of the pixels may be set to a relatively short value to avoid saturation in the image. However, the non-integration time (NIT), also referred to as non-exposure time, becomes longer if frame time is a constant—as is typical for video cameras. Since pixels continue to collect electrons even during NIT time, the pixels may become saturated and bloom into neighboring pixels.
If some of a scene includes bright regions adjacent to dark regions, the blooming from the bright pixels into the dark region may be relatively visible resulting in decreased image quality.